1. Field of the Invention
The present invention is directed to power amplifiers, and more particularly to power amplifiers that provide for local power feedback.
2. Background
Saturated RF power amplifiers can use two methods of internal power control: the voltage control approach and the current control approach. The voltage control approach sets the supply voltage at the output of the amplifier and uses that to control the output power. The advantage is that the amplifier chain and output stage remains in saturation. These designs are relatively inexpensive and can provide adequate amplification levels. However, they do have some disadvantages. Power delivered to the load depends on the load impedance and increases proportional to uncontrolled current.
Current controlled saturated RF power amplifiers are also used. In this case a regulated current into the load provides equivalent control under a nominal load. In this case, the power varies with voltage of the load, uncontrolled in a current controlled system.
A current controlled amplifier with an extra parallel load matched to the nominal load impedance would provide power control, but with serious waste of power in the parallel resistor.
A voltage controlled amplifier with a series resistor equal to the nominal load would be equivalent at small signal, but could never achieve low drop out voltage at the load end of the resistor.
Power amplifiers described above fail to provide for local power feedback, fail to have improved power control under varying loads, or are capable of maintaining an LDO low dropout during changes in the output impedance seen by the power amplifier.